U.S. patent number 4,213,158 [Application Number 05/920,314] was granted by the patent office on 1980-07-15 for optical data recording system utilizing acoustic pulse imaging to minimize image blur.
This patent grant is currently assigned to Xerox Corporation. Invention is credited to Leonard C. DeBenedictis.
United States Patent |
4,213,158 |
DeBenedictis |
July 15, 1980 |
Optical data recording system utilizing acoustic pulse imaging to
minimize image blur
Abstract
An optical data recording system in which during recording the
image of the acoustic pulse at the writing surface is made to move
at the same relative velocity with respect to the recording medium
whereby motion blur is minimized or reduced. The writing beam, such
as that generated by a laser, is incident on acousto-optic device
(such as a Bragg cell) and may be diffracted (deflected) at an
angle determined by the frequency of a source applied to the
device. By selecting the system magnification, in one embodiment,
between the device and the recording medium such that the
magnification is substantially equal to the ratio of the velocity
of the recording medium, to the velocity of the sound wave in the
acousto-optic device, the image of the acoustic pulse follows the
surface of the recording medium and permits imaging of the video
signal to the recording medium without blurring. In a second
embodiment wherein the writing beam is scanned across the surface
of the recording medium, the system magnification is selected to be
substantially equal to the ratio of the velocity of the writing
beam to the velocity of the sound wave in the acousto-optic
device.
Inventors: |
DeBenedictis; Leonard C. (Los
Angeles, CA) |
Assignee: |
Xerox Corporation (Stamford,
CT)
|
Family
ID: |
25443552 |
Appl.
No.: |
05/920,314 |
Filed: |
June 28, 1978 |
Current U.S.
Class: |
358/296; 347/255;
347/261; 358/474; 369/112.02; 386/222; G9B/7.002; G9B/7.105 |
Current CPC
Class: |
G11B
7/0025 (20130101); G11B 7/128 (20130101); H04N
1/036 (20130101); H04N 1/1135 (20130101) |
Current International
Class: |
G11B
7/00 (20060101); G11B 7/125 (20060101); G11B
7/0025 (20060101); H04N 1/036 (20060101); H04N
1/113 (20060101); G11B 007/00 (); H04N
005/76 () |
Field of
Search: |
;358/199,201,206,235,296,285,302,128 ;365/106,120,127,215,234
;346/108 ;179/1.3G,1.3V |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Okolicsanyi, "The Wave-Slot, an Optical Television System," The
Wireless Engineer, vol. 14, pp. 527-536, Oct. 1977. .
Robinson, "The Supersonic Light Control and Its App. to TV
w/Special Ref. & Scophony TV Receiver," Proceedings of IRE.;
pp. 483-486, Aug. 1939. .
Korpel et al., "A TV Display Using Acoustic Deflection &
Modulation of Coherent Light," Applied Optics, vol. 5, No. 10, pp.
1667-1675, Oct. 1966. .
Chang, "Acoustooptic Devices and Applications, "IEEE Transactions
on Sonics and Ultrasonics, vol. SU-23, No. 1, Jan. 1976, p.
14..
|
Primary Examiner: Tupper; Robert S.
Assistant Examiner: McElheny, Jr.; Donald
Attorney, Agent or Firm: Keschner; Irving
Claims
What is claimed is:
1. An optical data recording system for recording information on a
light sensitive member comprising:
means for providing a beam of radiant energy;
a Bragg diffraction light-sound interaction medium including a
transducer coupled to an acoustic wave transmitting medium
characterized by a predetermined acoustic wave propagation
velocity,
scanning means having at least one reflective surface positioned in
the optical path of said beam for scanning said beam across said
member by rotating said reflective surface a desired angle to
impart the information content of said beam to said member;
a video modulating signal source coupled to said transducer for
propagating intensity-modulated acoustic waves in said medium at a
predetermined velocity,
means for projecting through said medium in a direction transverse
to said acoustic waves said light beam to produce in said medium a
moving image of information corresponding to said modulating
signal, said image moving at said predetermined velocity; and
optical means interposed in the path of said intensity modulated
beam and having magnification M associated therewith, said moving
image being projected onto said scanned member within the envelope
of said beam in a manner such that the velocity of said moving
image is substantially equal to and in an opposite direction as the
velocity of said scanning beam, the direction of movement of said
beam and said image being substantially orthogonal to the direction
of rotation of said medium.
2. The optical data recorder as defined in claim 1 wherein said
medium comprises a rotating xerographic member and said scanning
means comprises a rotating polygon for scanning said moving image
and beam across said medium in a direction orthogonal to the
direction of rotation of said medium.
3. The optical data recorder as defined in claim 1 wherein the
magnification M is selected such that M is substantially equal to
V.sub.3 /V.sub.1 wherein M is the system magnification between the
interaction medium and said member, V.sub.3 is the velocity of said
scanned light beam and V.sub.1 is said predetermined velocity.
4. The optical data recorder as defined in claim 3 wherein the
magnification M is the magnification in a direction parallel to the
direction of scan.
5. An optical data recorder system for recording information on a
light sensitive member comprising:
means for providing a beam of radiant energy;
a Bragg diffraction light-sound interaction medium including a
transducer coupled to an acoustic wave transmitting medium
characterized by a predetermined acoustic wave propagation
velocity;
means for projecting through said medium in a direction transverse
to said acoustic waves said light beam to produce in said medium a
moving image of information corresponding to said modulating
signal, said image moving at said predetermined velocity; and
optical means interposed in the path of said intensity modulated
beam and having magnification M associated therewith, said moving
image being projected onto discrete areas of said member within the
envelope of said beam in a manner such that the velocity of said
moving image is substantially equal to and in the same direction as
the tangential component of motion of said discrete areas of said
member.
6. The optical recording system as defined in claim 5 wherein said
member comprises a rotating optical disc.
7. The optical recording system as defined in claim 6 wherein the
magnification M is selected such that M is substantially equal to
V.sub.3 /V.sub.1 wherein M is the system magnification between the
medium and said rotating member, V.sub.3 is said tangential
velocity and V.sub.1 is said prdetermined velocity.
Description
BACKGROUND OF THE INVENTION
Acousto-optic modulators have been utilized in prior art optical
recorders for recording information on recording mediums sensitive
to laser flux as shown for example, in U.S. Pat. No. 3,922,485. In
particular, this patent discloses a multifaceted polygon optical
scanner which scans a modulated laser beam across a xeorgraphic
medium. The beam may be modulated by an acousto-optic modulator
which is driven by a system which has, an one input, video input
information which is to be reproduced. A motion blur problem can
arise in those forms of optical data recorders in which there is
significant relative movement between the recording medium and the
focused writing beam. Reduction of motion blur by the use of very
fast electro-optic modulators is possible, but that technique tends
to be rather costly. State-of-the-art acousto-optic modulators are
not effective in many potential applications because of the
practical limitations in the rise time of the modulator which is
imposed by the transit time of the acoustic wave front across the
laser beam, thereby reducing or severely limiting the response of
the modulator to high speed input video information.
Korpel U.S. Pat. No. 3,514,534 discloses a laser modulating and
scanning system which utilizes a pair of acousto-optic devices to
modulate and deflect a laser beam across an image screen. By
positioning the acoustooptic devices apart a predetermined
distance, a visible replica of the video information to be
reproduced is formed in a manner such that the picture elements are
immobilized on the screen.
An article by D. M. Robinson, "The Supersonic Light Control and its
Application To Television with Special Reference to the Scophony
Television Receiver", proceedings of the I.R.E., vol. 27, pp.
483-486, August, 1939 discloses a system where a sound wave carries
with it a replica of the video signal received during an
immediately proceding time interval and which is projected on a
screen, a mirror polygon being utilized to move the whole image
across the screen at the same speed in the opposite direction to
immobilize the details on the screen.
What is desired is to adapt the concept of providing a replica of
the video signal carried by a sound wave to an optical recording
system which utilizes rotating devices, such a a xerographic drum
or an optical disc, as the reproduction medium and a simplified
technique for immobilizing the image of the acoustic pulse at
discrete areas at the writing surface to minimize image blur.
SUMMARY OF THE PRESENT INVENTION
The present invention provides a method for substantially reducing
the bandwidth and rise time limitations associated with the use of
state-of-the-art acousto-optic modulators in an optical data
recording system by reimaging the motion of the acousto-optic pulse
onto a recording medium thereby greatly increasing the effective
bandwidth of the acoustooptic modulator and reducing any blurring
of the image formed on the surface of the recording medium. In
particular, the laser beam incident on the acousto-optic modulator
may be diffracted, the diffracted beam being incident on the
recording medium. By selecting the system magnification between the
modulator and the recording medium to be substantially equal to the
ratio of the surface velocity of the recording medium, in a first
embodiment when the writing beam is fixed as it scans across the
recording medium, to the velocity of the acoustic wave front in the
acoustooptic modulator, the acoustic pulse (which essentially
contains the video information) is reimaged onto the surface of the
recording medium in a manner whereby the acoustic pulse follows the
recording surface and permits an isomorphic mapping of the video
signal to the recording medium without blurring. In a second
embodiment wherein the writing beam is deflected in a scanning
motion across the surface of the recording medium, the system
magnification is selected to be substantially equal to the ratio of
the velocity of the moving laser beam to the velocity of the sound
wave in the acousto-optic modulator. In essence, the present
invention utilizes the capability of an acousto-optic Bragg cell to
modulate the spatial profile of an incident light beam (in addition
to its well-known capability to modulate in time the power of the
light beam) to minimize image blur by tracking the surface of the
recording medium with a moving image of the video signal
stream.
It is an object of the present invention to provide an improved
optical data recording system.
It is an object of the invention to provide a scanning system which
incorporates an acousto-optic modulator, the acousto-optic
modulator being operated in a manner in which the bandwidth
requirements of the modulator are substantially reduced.
It is a further object of the present invention to provide a
technique for utilization of an acousto-optic modulator in a laser
scanning system wherein the bandwidth requirements of the modulator
are substantially reduced.
It is still a further object of the present invention to provide a
pulse imaging technique for use in a laser beam writing system
wherein the laser beam incident on the acousto-optic modulator
interacts with the acoustic pulse, the laser output beam being
projected onto a recording surface. Proper selection of the system
magnification between the modulator and the recording surface
provides a pulse image on the recording surface, the relative
velocity of the pulse image with respect to the velocity at the
recording surface being substantially zero thereby minimizing image
blurring on the surface of the recording medium.
DESCRIPTION OF THE DRAWINGS
For a better understanding of the invention, as well as other
objects and features thereof, reference is made to the following
description which is to be read in conjunction with the
accompanying drawings wherein:
FIG. 1 is a partial schematic diagram of one embodiment of the
optical scanning system of the present invention at the start of
scan position;
FIG. 2 is a schematic diagram of the active optical element used in
the present invention;
FIG. 3 shows in a simplified representation of the present
invention wherein, in the case illustrated, two acoustic pulses are
illuminated by an input laser beam and imaged onto an optical data
recording surface;
FIG. 4, including 4A-4C, illustrates the formation of transformed
video pulses as optical pulses on the surface of a recording
medium;
FIG. 5 shows in a simplified representation an optical disc which
can be utilized as the recording medium;
FIG. 6 shows a portion of the scanning system of the present
invention shown in FIG. 1 being utilized to print information on a
laser flux sensitive medium; and
FIGS. 7-11 illustrate in a more visual form the principles of the
present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIG. 1, a partial schematic diagram of one
embodiment of the improved optical scanning system of the present
invention at the start of scan position is illustrated. The optical
portion of the schematic diagram shows the scanning system at the
beginning of a scan line 10, the scan line 10 being in the
direction of arrow 12 along the surface of a medium 14 which is
sensitive to the laser flux utilized in the system. It is assumed
that the scan line 10 starts at point 16 on the surface of medium
14, the scan line 10 being along a width x, i.e. from point 16 to
point 18. As shown, medium 14 is preferably a xerographic drum
which is rotated in the direction of arrow 19 to provide the Y
direction of scan. It should be noted at this point that the
recording medium may be an optical disc with the writing laser beam
directly incident on the optical disc without the necessity of a
scanning poloygon as will be described with reference to FIG. 5
hereinafter. A light source 20 provides the original light beam 41
for utilization by the scanning system. The light source 20
generates a collimated beam of light and preferably comprises a
laser, such as a helium-cadmium or helium-neon laser. The laser
which generates a collimated beam of monochromatic light may easily
be modulated by an active optical element, acousto-optic modulator
30, in conformance with the information contained in a video signal
applied to the modulator controller as will be set forth
hereinafter. A lens 21 is provided to focus the beam 41 onto the
modulator 30.
FIG. 2 illustrates in schematic form the active optical element
which may be used in the present invention. The element 30 is an
acoustooptic Bragg cell or, as it is more commonly called, an
acousto-optic modulator (hereinafter referred to as modulator). The
modulator 30 comprises an electrically driven piezoelectric
transducer 31, such as lithium nobate, bonded to an acousto-optic
material 33 which may be glass, plastic, or crystal such as a
single tellurium dioxide (TeO.sub.2) crystal. The transducer 31 in
response to an electrical drive signal generates an acoustic-wave
which travels through the material, perturbing the index of
refraction and acting as a phase grating 32, the grating period
being equal to the drive or acoustic frequency. Grating density
(ability of grating to modify the phase front of light beam) is
proportional to the amplitude of the drive signal applied to the
transducer 31. The wavefronts are segmented by the video signal
characteristics, and for a video signal comprising a stream of "1"s
and "0"s, it is assumed that the spacing between segmented
wavefronts, is determined by the "0" time of the video signal
although the "1" time can be utilized.
A beam of light 41 is applied to the modulator. Without a signal
applied to the transducer 31, only a non-diffracted output beam 43
exists. Application of a signal to the transducer from a fixed
frequency oscillator 54 produces two significant output beams, a
first-order diffracted beam 42 and a zero-order non-diffracted beam
43. In the present invention, the first-order beam is the output,
while the zero-order beam is absorbed by a beam stop 26 (the
zero-order beam may be utilized as the output if desired). The
intensity of the output beams is a function of the amplitude of the
drive signal applied to the transducer.
An angle .theta. which represents the approximate angle of
diffraction exists between the first-order and zero-order beams.
The angle .theta. is directly proportional to the drive frequency
f, the wave length of the incident laser light .lambda. and
inversely proportional to the velocity of propagation V of the
acoustic pulses in a modulator 30. An example of a modulator which
may be used in the present invention is disclosed in U.S. Pat. No.
3,938,881.
With reference again to FIG. 1, the first-order beam 42 is
positioned at the output of modulator 30 as shown, passes thorugh a
recollimating lens 22, and is then incident on cylindrical lens 23
having power in the tangential (direction of scan) direction.
Cylindrical lens 23 expands beam 42 into a beam 44 which is
incident on optical element 24 which has a magnification level
M.sub.1 associated therewith. In the embodiment shown, optical
element 24 comprises two elements, a biconcave element 46 and a
second convex lens 47 cemented to biconcave lens element 46.
Optical element 24 is configured to form an image of the acoustic
pulses in modulator 30, as set forth hereinbelow, onto the surface
of the recording medium 14. Although not shown, the magnification
M.sub.1 of optical element 24, which also may comprise a single
lens element, is selected to be variable over a predetermined range
in a manner known in the optical magnification art. The elements 23
and 24 and the distance therebetween are selected such that the
beam portion 48 at the output of optical element 24 is directly
incident onto a facet 28 of rotating multifaceted polygon 27 and
thereafter focused via cylindrical lens 25 as scan line 10
proximate the surface of recording medium 14. Facet 28 may be fully
illuminated along its width by the beam 48 as shown or the beam 48
can be compressed to a small spot on the facet 28.
The rotational axis of polygon 27 is orthogonal or nearly
orthogonal to the plane in which light beams 42 travels. The facets
of polygon 27 are mirrored surfaces for the reflection of any
illuminating light impinging upon them. With the rotation of the
polygon 27 in the direction shown by arrow 29, light beam 48 is
reflected from illuminated facet 28 and translated through a scan
angle for flying spot scanning.
The beam portion 50 reflected from facet 28 passes through
cylindrical lens 25 which has power only in the sagittal direction
(orthogonal to the direction of scan), beam portion 50 being
focused at point 16 on surface of medium 14 as shown.
The acousto-optic modulator 30 is used to modulate the light beam
41 in accordance with the information contained in the electrical
video signal supplied to the control circuit 52. In the control
circuit a fixed frequency oscillator 54 provides an output of
constant amplitude and constant frequency which is mixed with the
electrical video signal in mixer 56. The output of the mixer 56 is
amplified by amplifier 58 and then used as the drive signal to the
transducer 31. The output beam of the modulator 30 may be either
the zero-order beam or the first-order beam, the intensity of
either beam being a function of the amplitude of the drive signal
applied to the transducer 31.
FIG. 3 is a simplified representation which sets forth the
particular features of the present invention. In particular, the
input laser beam 41 is focused onto Bragg cell 30 and the video
information is impressed upon the r-f carrier which drives the
travelling wave Bragg cell in a manner described hereinabove. As is
well known in the art, acoustic pulses 100, 102, 104 and 106. . .
are set up in the modulator 30 corresponding to an acoustic volume
phase grating, the spacing between the wavefronts being
proportional to the input signal which may be a digitized scanned
input, analog video signal or from a source of data, such as a
computer. In the case of a binary signal the spacing between the
wavefront a, b, c . . . is proportional to the duration of a "0"
input signal as shown in FIG. 4(a) which may in turn correspond to
the printing of no information on the laser sensitive medium. In
effect, the video information (video pulses 100, 102 . . .
corrrespond to the acoustic pulses 100, 102 . . . ) is transformed
into coded segments of the acoustic volume phase grating produced
by the data, or video, modulated RF carrier signal. Modulator 30 is
oriented with respect to other system elements (such as folding
mirrors which are not shown) so that the acoustic field travels in
the proper direction with respect to the surface of the recording
medium. In the case of the FIG. 1 embodiment, the acoustic field
travels anti-parallel to the scan direction (or optical equivalent
thereof). In other words, the acoustic field moves anti-parallel to
the direction that the writing laser beam is caused to move
relative to the recording medium. In the situation wherein an
optical disc is utilized as the recording medium, the acoustic
field travels in the same (parallel) direction as the velocity
vector at the discrete area of the recording surface which is to be
recorded on. Incoming video beam 41, in the case illustrated, has a
sufficient width in the direction of sound propagation, indicated
by the arrow labeled V.sub.1, representing the sound wave velocity,
to encompass two coded segments or "bits" (102 and 104) of the
acoustic volume phase grating although more or less coded segments
could be illuminated. The more coded segments illuminated, the
better the resolution of the data recorded. Preferably, between one
and two coded segments are illuminated. The coded segments 102 and
104 transform the incident laser beam 41 into separate optical
beams 110 and 112, respectively, which move at the velocity of the
phase grating within modulator 30. The optical element 24 is
positioned relative to acousto-optic modulator 30 such that the
deflected beams 110 and 112 are incident thereon and imaged as
pulses 120 and 122, respectively, onto the surface of recording
medium 140.
The element 140 shown in FIG. 3 represents the surface of a medium
sensitive to laser flux incident thereon and may represent, for
example, an optical disc or a xerographic member, such as
xerographic drum. In the case of xerographic drum, the velocity of
the recording medium 140 at the area wherein a scanning laser beam
may be incident is essentially zero in the direction of scan
(reference numeral 12 shown in FIG. 1). In the case wherein medium
140 is an optical disc, the velocity of the disc at the discrete
area wherein the laser beam is incident (as will be described in
more detail hereinafter) is substantially equal to the tangential
velocity of the disc at that area. FIGS. 4(b) and 4(c) further
illustrate the principles of the present invention. FIG. 4(b) shows
in enlarged form a portion of the scanline 10 being formed on
xerographic drum 14 and the beam 50 imaged thereon. The beam 50 at
the surface 140 at the time when the acoustic wavefront interacts
with the laser beam comprises coded optical pulses 120 and 122, the
individual wavefronts in the modulator 30 not being resolved since
the first order light is blurred to some extent. The spacing d
between pulses 120 and 122 is proportional to the spacing between
acoustic pulses 102 and 104 which in turn is equal to the product
of the velocity of sound in the modulator 30 to the time interval
of the marking video pulse. The width of the pulses 120 and 122 in
the sagittal direction is determined by the shape of the laser beam
interacting with the sheet of sound formed in the acousto-optic
medium and the magnification of optical element 24 and lens
elements 22, 23 and 25 in the sagittal direction. The width of the
sound sheet 151, the laser beam shape, or envelope 153 and the
acoustic pulses 120 and 122, as scaled by the magnification
provided by the optical elements between modulator 30 and medium
10, is illustrated in FIG. 4(b). Since the beam 50 is caused to
scan xerographic medium 10 in the direction of scanline 12 at a
velocity V.sub.3, the velocity of the drum in that direction being
essentially zero, it is required that the pulses 120 and 122 move
in the opposite direction (reference numeral 13) at a speed V.sub.4
such that the image formed on the drum appears stationary, or
immobilized, as the information is being printed on the drum in
order to minimize image blur. In this regard, modulator 30 is
oriented so that the image of the acoustic field (pulses) which is
projected onto the surface of medium 14 travels in a direction 13
antiparallel to the scan motion introduced by the rotating polygon
mirror 27. Although not illustrated, as the laser beam 50 continues
to scan across the surface of xerographic drum 14, additional
acoustic pulses will be imaged onto the surface of xerographicdrum
14 synchronized to the video information to be reproduced, an
individual scanline thereby being formed. Additional scan lines are
formed in accordance with video information to be reproduced using
known scanning techniques.
It should be noted that FIG. 4(b) (and FIG. 4(c) to be described
hereinbelow) illustrate the situation wherein the video pulse is of
a relatively short duration (i.e. 10 nanoseconds) such that the
optical pulses or segments 120 and 122 are formed within the
envelope 153 of the laser beam. If the video pulse is of a longer
duration such that the width of the corresponding optical pulse
segment extends beyond the envelope 153, the acousto-optic pulse
imaging system of the present invention still provides the desired
results since the marking cycle, when completed, will provide the
same exposure or mark due to the time of exposure which occurs.
Referring to FIG. 4(c), the same optical beam 50 is shown incident
(in an enlarged form) on a discrete area of a particular track 154
of optical disc 156. The use of optical discs for recording
information by utilizing a laser beam impinging thereon is known in
the prior art. In this configuration, the velocity of the laser
beam at the surface of the optical disc is essentially fixed at the
time data is to be recorded since in optical disc recording
technology, the laser is generally positioned to a desired track,
the laser then being energized at the appropriate time as the
optical disc rotates therepast. In this case, it is required that
the velocity V.sub.4 of the optical pulses 120 and 122 (in the
direction of arrow 15) be equal to and in the same direction
(reference numeral 17) as the velocity V.sub.4 of the area of the
track whereat information is to be recorded. In this case, the
velocity of the discrete track area is essentially equal to the
tangential velocity V.sub.4 of the disc at that discrete area of
the track.
In both configurations shown in FIGS. 4(b) and 4(c), the optical
pulses 120 and 122 are shown at a particular instant of time,
additional optical pulses being produced as the acoustic pulses are
generated in medium 30.
Referring to FIG. 3, and assuming that the recording medium 140 is
an optical disc, each of the coded segments 102 and 104 produce
corresponding optical pulses 120 and 122, respectively, the
separation between the acousto-optic pulse images corresponding to
the separation between the segmented coded pulses 102 and 104. In
essence, the light output from modulator 30 is broken up into
spatial instead of temporal segments.
Acousto-optic interaction occuring in the region of acoustic pulses
102 and 104 cause the input light to be diffracted, the
undiffracted or zero order light being absorbed, in the embodiment
illustrated, by zero order stop member 26. The first order
diffracted light is diffracted by the moving acoustic grating to
optical element 24 which projects the optical beams 110 and 112
onto medium 140 as optical pulses 120 and 122, respectively.
According to one of the principles of the present invention, the
recording medium selected is a xerographic medium as shown in FIG.
1 wherein the scanning direction is orthogonal to the direction of
rotation of the drum. If the system magnification M between the
acousto-optic modulator 30 and the surface of the photoreceptor is
selected such that -MV.sub.1 =V.sub.3 wherein V.sub.1 is the
acoustic velocity in the medium 30, V.sub.3 is the relative
velocity of the laser scanning beam in the direction of scan
(velocity effects in the direction of drum rotation are negligible)
which can be measured, for example, by utilizing appropriate start
and end of scan detectors, the image 120 and 122 of acoustic pulses
102 and 104, respectively, follows the velocity of the scanning
beam in the opposite direction (relative velocity MV.sub.1 of
imaged acoustic pulses 120 and 122 with respect to the
photoreceptor is substantially zero) and thereby permits an
isomorphic mapping of the video signal to the surface of the
recording medium with minimized blurring since the velocity of the
medium in the direction of scan is substantially zero. The minus
sign in front of the above relationship indicates that the optical
elements between the acousto-optic modulator 30 and the surface of
the recording medium should be selected such that the pulses are
travelling, in the appropriate sequence, in a direction opposite to
the scanning direction as exemplified in FIG. 4(b). It should be
noted that in the actual system, each of the optical elements may
contribute to system magnification other than optical element 24.
The system of the present invention is designed so that the system
optical magnification is of the proper value to make pulses 120 and
122 immobilized on the surface of the recording medium. A technique
to ensure that the system optical magnification, after the system
has been built, is of the proper value is to monitor the contrast
ratio of the recorded images while adjusting lens magnification. To
exemplify the above relationship, the acoustic compressional wave
velocity V.sub.1 may be calculated to be approximately
4.25.times.10.sup.5 cm/sec for a Te0.sub.2 acousto-optic modulator.
For a scanning beam velocity of 2500 cm/sec, the system
magnification between modulator 30 and the recording surface should
be: ##EQU1## Therefore, the velocity of the pulse images 120 and
122 at the recording medium surface is approximately 2500 cm/sec.
It should be noted that it has been determined that the best
performance of the system occurs when V.sub.3 /V.sub.1 =-M (or M
for the case of the optic disc recording medium as described
hereinbelow). However, it has been further determined that even if
the magnification M can not be adjusted exactly to that ratio but
is within around 10% of that value, the resolution characteristics
of the optical data recording system will still be improved over
the uncompensated system. This is particularly pertinent to the
optical disc recording embodiment since the velocity V.sub.3
utilized in the above equation is selected to correspond to the
average velocity between the outermost and innermost recording
tracks and one system magnification only may be provided.
A conventional optical memory device 156 may be utilized as the
recording medium and as shown in FIG. 5 is comprised of a substrate
disk 158 having on one surface thereof a storage or recording
medium 160 in the form of a thin film, such as, for example, a film
of bismuth on the order of 500 A thick. During system operation,
the optical memory is rotated at a constant speed by means of a
drive motor. Modulated laser light beam 170, produced in a manner
described hereinabove with reference to FIG. 3, produces a change
in the optical characteristics of discrete, closely spaced portions
172 and 173 of recording medium 164 thereby providing a record of
the information conveyed by the data signal supplied to modulator
30. Portions 172 and 173 are produced by the transformed video
pulses 120 and 122 as described with reference to FIGS. 3 and 4
hereinabove.
It should be noted that the image velocity vector, which is a
change of length in a unit time, can be increased or decreased
linearly by the magnification factor M. Further, the magnification
of an optical system can be different in the sagittal direction
(direction orthogonal to direction of scan) and tangential
direction (direction parallel to direction of scan). Since the
blurring effect would be most apparent in the direction of scan,
the magnification relationship set forth hereinabove is for the
direction of scan.
As noted, a problem of motion blur can occur whenever writing a
stream of high density data bits on a moving medium. In the case of
optical data recording, such as recording on optical memory device
156, practically attainable rise and fall times of modulator 30 are
often not short enough to produce the short pulses required to
produce the desired hole (bit) size, thus causing significant loss
of writing spot definition due to the movement of optical memory
device 156 during the writing or recording of information, this
relative movement causing blurring of the recorded data. Even if
the required modulator was available, potentially serious growth of
spot size can occur as a result of recording medium motion. Another
disadvantage in being required to provide extremely short laser
pulse widths (i.e. a short duty cycle, duty cycle being defined as
the ratio of the laser pulse width to the repetition width) in
order to minimize image blur as would be required in prior art
systems would be that the amount of energy coupled to the disc
surface would be reduced, resulting in the necessity of providing
lasers having higher power capability which increases overall
system cost. Further, the prior art systems utilizing continuous
wave lasers would be inefficient since the laser beam is utilized
for a small portion of the laser on time.
In accordance with the invention, motion blur is eliminated or
minimized by the use of optical element 24 in the optical path
between modulator 30 and the surface of recording medium 156 as
described with reference to FIG. 1. The data is recorded on
recording medium 156 in such a way that the image of the acoustic
pulses 120 and 122 at track 174, for example, moves at
approximately the same velocity as the recording medium area to be
recorded on (track 174). After an appropriate time, the writing
spot can be turned off by modulator 30 for the next "off" bit.
Typical bit spacing may be on the order of 1 .mu.m, and a typical
bit size may be 0.5 .mu.m to 1 .mu.m. The optical disc may be
recorded on by using ablationtype techniques or recording by
changing the optical characteristics of the recording medium by
means other than by ablation.
In particular, information is generally recorded on optical or
video discs on concentric tracks 174, 176 . . . 180. The radial
distance between the innermost track 174 (radius r.sub.1) and the
outermost track 180 (radius r.sub.2) may be on the order of 2
inches. Since the tangential velocity of the optical disc at each
track is proportional to the radius of the track from the track to
the disc center, the magnification M is adjusted, in the preferred
embodiment, to be equal to the ratio of the tangential velocity
corresponding to a radius midway between the inner and outer radius
r.sub.1 and r.sub.2 respectively and the velocity of the pulses in
the acousto-optic medium 30. Thus, although the velocity term in
the equation set forth hereinabove will not correspond exactly to
the desired value, as the laser beam is positioned within the disc
recording area the compensation provided is sufficient to minimize
image blur. It should be noted, however, that means may be provided
to automatically adjust the magnification of optical element 24 to
equal the desired value as the laser beam is positioned within a
disc recording area.
In accordance with the teachings of the present invention, it has
been recognized that the video signal information which is required
at the surface of the recording medium already exists within the
acousto-optic modulator. In particular, the usual perception of an
acousto-optic Bragg cell is that of a device which solely modulates
in time the power of a light beam. The acousto-optic Bragg cell
also modulates the spatial profile of the light beam. This latter
capability minimizes image blur by tracking the moving recording
surface with a moving image of the video signal stream. The spatial
modulation is defined by the overlap of the light beam profile with
the moving acoustic video signal stream (in essence, the packets of
sound energy inside the modulator constitute a series of "windows"
flowing past the light beam, successively exposing various segments
of the laser light profile). This modulated light profile is then
imaged onto the recording surface via appropriate optical elements
so that the moving segments of light travel at the same speed as
the disc surface, no blur occuring since there will be no movement
of the light segment with respect to the disc surface. In order for
tracking to occur, the requirements set forth hereinabove must be
met. The acoustic video stream does not have the required
dimensional scaling but in all other respects it is a faithful
reproduction of the desired video image. The scaling is corrected
by imaging the acoustic pulses onto the recording surface with the
appropriate magnification provided by the optical elements
interposed between the acousto-optic modulator and the surface of
the recording medium.
FIG. 6 shows, in more detail, the development of an image formed on
a xerographic drum shown in FIG. 1. In particular, medium 10 may be
a xerographic drum which rotates consecutively through a charging
station depicted by corona discharge device 190, exposure station
192 where the beam from the rotating polygon 27 traverses the scan
width x on the drum 14, through developing station 194 depicted by
a cascade development enclosure, transfer station 196 where a web
of copy paper is passed in contact with the drum 14 and receives an
electrostatic discharge to induce a transfer of the developed image
from the drum 14 to the copy paper. The copy paper is supplied from
the supply reel 198, passes around guide rollers 200 and through
drive rollers 202 into receiving bin 204. A fusing device 206 fixes
the images to the copy paper as it passes to bin 204.
Usable images are provided in that the information content of the
scanning spot is represented by the modulated or variant intensity
of light respective to its position within the scan width x. As the
spot traverses the charged surface 192 through a given scan angle,
the spot dissipates the electrostatic charge in accordance with its
light intensity. The electrostatic charge pattern thus produced is
developed in the developing station 194 and then transferred to the
final copy paper. The xerographic drum 14 is cleaned by some
cleaning device such as a rotating brush 208 before being recharged
by charging device 190. The polygon 27 is continuously driven by
motor 210 and synchronized in rotation to a synchronization signal
representative of the scan rate used to obtain the original video
signal. The rotation rate of the xerographic drum 14 determines the
spacing of the scan lines. It also may be preferable to synchronize
the drum 14 in some manner to the signal source to maintain image
linearity.
Another significant advantage of utilizing pulse imaging scanning
over prior art scanners concerns appropriate illumination of the
limiting aperture to achieve maximum resolution. The limiting
aperture in the embodiment of FIG. 1 is the polygon facet 28. The
limiting aperture in the embodiment of FIG. 5 is an imaging element
(not shown). Standard scanner design theory teaches that resolution
performance is governed by the convolution of the video signal
stream with "impulse response" of the scanner. This "impulse
response" is the spatial intensity profile of the scan spot (16 in
FIG. 1). Optimum resolution occurs when this spot is most compact.
The scan spot will be most compact when the limiting aperture
(facet 28 in FIG. 1) is uniformly illuminated.
Because the laser beam profile is not uniform, but typically
Gaussian, the uniform illumination of the limiting aperture can
only be approximated by overfilling the limiting aperture with
laser light as shown, for example, in the aforementioned U.S. Pat.
No. 3,922,485. However, only a fraction of the light energy,
typically less than 50%, falls within the limiting aperture of the
scanner optics. Therefore, the light throughput efficiency of the
scanner cannot be better than 50%.
In contradistinction thereto, the pulse imaging scanner of the
present invention exhibits its best resolution performance when the
light beam incident upon the limiting aperture subtends only a
fraction of this aperture i.e. an underfilled condition. This
configuration has inherently high light capture at the limiting
aperture, and hence significantly greater light throughput
efficiency. In particular, resolution does not degrade because of
FM blur wherein the light intensity profile grows in width with
more rapid video signal fluxtuations.
Because of FM blur, the limiting aperture can be underfilled for
quiescent video signals, and filled for rapidly varying video
signals. This allows a far more favorable tradeoff between light
capture and resolution. In particular, the resolution performance
of the pulse imaging scanner is governed by the relative
dimensional scaling between the quiescent (steady state video
signal applied to the modulator) light profile and the limiting
aperture. If the quiescent light severly overfills the aperture (a
low light throughput efficiency configuration), then the resolution
performance will be equal to the prior art scanners. In the
opposite extreme, when the quiescent light profile subtends a small
fraction of the aperture, then resolution performance is increased
greatly. Therefore, the pulse imaging scanner provides a range of
video signal operating frequencies wherein resolution is greatly
improved over the prior art scanners and wherein its performance is
at least equal to the prior art scanners at higher operating
ranges.
In order to more clearly visualize the pulse imaging concept of the
present invention, reference is made to FIGS. 7-11 (for the purpose
of this illustration, the optical disc is utilized as the recording
medium). FIG. 7 illustrates the overlap (shaded area) of the
incident laser beam and the acoustic video stream in successive
time frames a,b,c, and d as would be seen by an observer located at
modulator 30. FIG. 8 illustrates, in the same successive time
frame, the spatial profile of the modulated light beam as seen by
an observer located at modulator 30. FIGS. 9 and 10 illustrate the
writing beam profile in successive time frames as seen by an
observer located on the surface of an optical disc, FIG. 9 also
illustrating how the writing beam movement tracks the optical disc
motion. FIG. 11 illustrates the resulting (composite) idealized
exposure profile formed at discrete areas on the surface of the
optical disc.
While the invention has been described with reference to its
preferred embodiments, it will be understood by those skilled in
the art that various changes may be made and equivalents may be
substituted for elements thereof without departing from the true
spirit and scope of the present invention.
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